The present invention relates to an antenna, and more particularly to an improved antenna for use with radio frequency identification (RFID) tags. Such tags can be used in many applications including tracking goods in transit or within a production facility, or for use in anti-pilfering systems to protect articles from shoplifting.
A problem with such tags, which are designed to be used globally, is that it is difficult to design a tag that is suitable for use in all regions of the world without a license. This is due to the differing regulation regimes operating in different countries.
It would be desirable to implement a passive wide band tag that operates on the Industrial Scientific and Medical (ISM) band for both the US (902–928 MHz) and Europe (869.40–869.65MHz). Different regulations govern the performance of devices operating in the ISM band in different countries. In the USA, a maximum transmit power of 4W (EIRP—Equivalent Isotropically Radiated Power) is allowed, and in Europe, a maximum transmit power of 0.5W (ERP—Effective Radiated Power) is allowed. The difference in the form of the maximum allowed in each region is derived from a different reference source. The USA reference is an ideal isotropic radiator, and the European reference is a dipole.
Passive tags can only radiate when interrogated as they rely on received power to generate a transmission. Active tags, however, can potentially radiate at all times and thus may interfere with other devices in their vicinity.
In order to operate on both bands for the USA and Europe, it is desirable to have a wide band antenna with relatively high gain and small size.